VGhippy on 16 July 2013
Well Microsoft does produce a lot of PC games, like Age of Empires, Mechwarrior and Microsoft Flight, and they do have Games for Windows Live exclusive to several games like the Arkham Asylum/City series so you have to use it just to play them (even if you buy them on Steam). Obviously the entirety of PC gaming is not part of the MS ecosystem, but they are still reasonably large players.
